In a conventional process, a grain oriented electrical silicon steel sheet containing aluminium is produced in the following manner.
A melt of silicon steel which has been produced by using a conventional steel-making furnace is cast to form an ingot by a conventional ingot method or a slab by a continuous casting method, and in the case of a formed ingot the ingot is converted into a slab, if necessary. The slab is hot rolled and, then, cold rolled once or twice to form a steel sheet having a desired dimension. The hot rolled sheet is composed of 2.5 to 4.0% by weight of silicon, 0.02 to 0.085% by weight of carbon, 0.010 to 0.06% by weight of aluminium, 0.002 to 0.010% of nitrogen and the balance consisting of iron and inevitable impurities. The steel sheet may contain a desired amount of mangnese and sulfur and, optionally, a small amount of Se, Te, Sb, Sn, Pb, V, Cr, Ni, Cu and/or B. Before the above-mentioned cold rolling operation, the hot rolled steel sheet may be subjected to an intermediate annealing operation in order to allow AlN to be uniformly distributed and precipitated in the steel sheet. The cold rolled steel sheet is subjected to a decarburization operation, shaped into a desired shape, usually, the shape of a coil, coated with an annealing separator comprising MgO and, then, subjected to a batch type final annealing operation in which the secondary recrystallization of the steel sheet occurs.
Attempts have been made to produce an electrical silicon steel sheet having an extremely high magnetic flux density B.sub.8 of 1.90 or more at a magnetizing force of 800 A/m, by applying such a secondary recrystallization annealing operation that the silicon steel sheet is heated in a reducing atmosphere containing a predetermined amount of nitrogen until the temperature of the reducing atmosphere reaches a level at which the secondary recrystallization of the steel sheet is completed.
Japanese patent application laying-open (Kokai) No. 50-134917 discloses another method for increasing the magnetic flux density of the steel sheet during the final annealing operation. In this method, the dew point of the annealing atmosphere is adjusted so that it is within a special range.
The inventors of the present invention have studied the above-mentioned prior art methods and, based on those methods, have created an improved method for producing an electrical silicon steel sheet having an extremely high magnetic flux density. This is, the inventors of the present invention have studied in detail the secondary recrystallization behaviour of the steel sheet in relation to the precipitation and disolution behaviour of AlN during the final annealing operation. As a result of their study, it was discovered by the inventors that, in the case of a steel sheet which has been completely secondary recystallized and yet has a poor magnetic flux density, the crystal grains in the steel sheet just before the onset of secondary recrystallization are substantially uniformly distributed in the direction normal to the plane surfaces of the steel sheet. However, in the case of a secondary recrystallized steel sheet having an excellent magnetic flux density, it was found that the crystal grains located in the surface layer of the steel sheet had grown into coarse crystal grains just before the onset of secondary recrystallization and the nuclei of the secondary crystals were created in bounding regions between the course crystal grains in the surface layer and fine crystal grains in the inside layer of the steel sheet. Also, it was discovered by the inventors that the thicker the layer of the coarse crystal grains, the farther the secondary crystal nuclei are located from the outside surface of the steel sheet. As long as the location of the secondary crystal nuclei is within about 60 to 80 microns from the outside surface of the steel sheet, then the farther the secondary crystal nuclei are located from the outside surface of the steel sheet, the higher the magnetic flux density of the resultant secondary recrystallized steel sheet. However, when the location of the secondary crystal nuclei is about 90 microns or more from the outside surface of the steel sheet, it becomes difficult to cause secondary recrystallization. That is, during the secondary recrystalling operation, the so-called fine crystal grains are generated in the steel sheet, and the resultant steel sheet exhibits a remarkably poor magnetic flux density. Furthermore, it was discovered by the inventors that, since the coarsening of the crystal grains located in the surface layer of the steel sheet is promoted by the reduction in concentration of AlN in the surface layer during the final annealing operation, the thickness of the coarse crystal grain layer can be controlled by controlling the concentration of AlN in the surface layer of the steel sheet during the final annealing operation. It is possible to promote a reduction in the concentration of AlN near the surface layer of the steel sheet during the secondary recrystallization annealing operation by slowing down the heating rate, however, the productivity in the final annealing has to be lowered. The present invention makes it possible to lower the concentration of AlN the surface layer even under a higher rate of heating than that of a conventional annealing process leading to the increase of productivity in the final annealing proces.
The present invention has been attained based on the above-mentioned discoveries.